statistical evaluation of elemental composition of humic substances

Org. Geochem. Vol. 17, No. 5, pp. 635-648, 1991 0146-6380/91 $3.00 + 0.00 Printed in Great Britain. All fights reserved Copyright © 1991 Pergamon Press plc

Statistical evaluation of the elemental composition of humic substances

JAMES A. RIC~* and PATRICK MACCARTHY Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401, U.S.A.

(Received 19 April 1990; accepted 7 December 1990)

Abstract--Elemental data (C, H, O, N, S, atomic H/C and O/C ratios) for humic acids (410 samples), fulvic acids (214 samples) and humin (26 samples), isolated from environments all over the world, were compiled from the literature. These data were analyzed statistically using the mean, median, mode, range, standard deviation and t-test. The large data base allows statistically significant differences between various humic substances to be established. Prior to some of the statistical evaluations, the humic substances were grouped according to source---soil, freshwater, marine or peat. The average elemental compositions for humic acid, fulvic acid and humin, in tow, and also when segregated by source, are presented. Standard deviations for carbon contents are remarkably small suggesting that perhaps an optimum composition exists for humic substances in nature. The evaluation shows that humic acids in tow are significantly (P < 0.0005) different from fulvic acids in tote with respect to C, N, and O contents and atomic H/C ratios. Fulvic acid in tote is significantly (P < 0.0005) different from humin/n tote in terms of C and O contents. When segregated by source, some significant (P < 0.0005) differences between humic acids isolated from freshwater, marine, and soil environments are evident; similarly, significant differences are found between fulvic acids from freshwater and soil sources. Three van Krevelen diagrams based on: (1) the 650 samples of humic acid, fulvic acid and humin; (2) humic acids segregated by source, and; (3) fulvic acids segregated by source, are discussed.

Key words--humic acid, fulvic acid, humin, elemental composition, statistical analysis

INTRODUCTION

One of the most fundamental characteristics of a chemical substance is its elemental composition. De- termining the elemental composition (or empirical formula) of a discrete chemical compound is a first step to obtaining its molecular formula; this, in turn, is critical for determining its structural formula. However, in the case of non-stoichiometric materials such as coal, kerogen and humic substances, the concept of even a net molecular formula is extremely limited, and a unique, structural formula does not exist for these materials.

Nevertheless, elemental analysis is a useful tool for characterizing non-stoichiometric mixtures such as humic substances. The applications of elemental analysis in the study of humic substances have re- cently been reviewed by Steelink (1985). Because humic substances are non-stoichiometric materials they must be characterized in terms of their average properties. Accordingly, it is appropriate to conduct a statistical evaluation of these properties in order to establish the statistically valid differences and trends in these materials. Elemental composition is one such average property and it is statistically evaluated in this paper.

*Present address: Department of Chemistry, South Dakota State University, Brookings, SD 57007, U.S.A.

van Krevelen (1961) developed a graphical method to study the coalification process in which the atomic hydrogen/carbon (H/C) ratio is plotted as a function of the atomic oxygen/carbon (O/C) ratio. This type of plot, now generally known as a van Krevelen diagram, is often used for the classification of coals and kerogens. A frequent application of the van Krevelen diagram is to illustrate the changes in elemental compositions that occur during the alter- ation of organic geochemicals in a geologic environment; e.g. H/C and O/C ratios have been used to follow the effects of diagenesis on humic substances (Huc and Durand, 1977; Reuter and Perdue, 1984). van Krevelen diagrams have also been used by various workers to illustrate compositional differences between humic acids and fulvic acids, and also to show variations in humic substances as a function of source. For example, Kuwatsuka et al. (1978) used a van Krevelen diagram to compare the elemental compositions of soil humic and fulvic acids, coals, plant tissues and various classes of organic com- pounds. Visser (1983) employed a van Krevelen diagram to compare fulvic and humic acids from aquatic and terrestrial sources.

The magnitude of the H/C ratio has also been used to indicate the degree of aromaticity or unsaturation (a small value) or aliphaticity (a large value) of a substance (van Krevelen, 1961). Perdue (1984) has pointed out that the total unsaturation of a humic

635

636 JAMES A. RJc'~ and P^rmCK M^CC~Th'V

material cannot be obtained solely from the H / C ratio; in addition to unsaturated forms of carbon the H/C ratio is also a function of unsaturation present in functional groups, primarily carboxyl and car- bonyl groups, with lesser contributions from other miscellaneous forms of unsaturation. If H/C ratios are calculated for the 21 humic material samples in the study of Perdue (1984) and compared to the aromatic carbon contents corrected for the various forms of noncarbon unsaturation it is seen that, though the actual numbers differ, samples which exhibit a high aromatic carbon content also exhibit a small H/C ratio and vice versa. The lone exception is a spodosol fulvic acid with high total acidity (12mequiv/g), a low corrected aromatic carbon content but a moderate H/C ratio (0.85). The H/C ratio thus appears to be a qualitatively useful parameter for comparing the aromaticities of humic materials.

To date, the number of humic samples plotted on a single van Krevelen diagram has been relatively small. The value of any such investigation would be enhanced by enlarging the data base and by using humic substances from a wider variety of source environments. In addition, when a large data set is employed one can justifiably apply statistical methods to quantify the relationships between the various groups of humic substances by establishing the statistically significant differences. For this study we compiled from the published literature an exten- sive data base consisting of the elemental contents of 650 samples of humic substances. The objectives of this paper are:

(1) to apply basic statistical methods to a very large data base of elemental contents in order to quantify the relationships between various classes of humic substances; and,

(2) to study van Krevelen diagrams for humic substances prepared from this large data base.

DATA AND DATA ANALYSIS

Elements studied Elemental contents (C, H, N, S, and O) of humic

acids, fulvic acids and humins were compiled from the literature. Elemental contents are generally reported on a weight/weight percentage basis and that is the form used in this paper. All elemental contents are expressed on an ash-free basis, and, where necessary, the ash-flee values were calculated by the present authors using the published ash-contents. Atomic H/C and atomic O/C ratios were computed for all samples and were plotted on a van Krevelen diagram. For this study, the samples were classified into humic acid, fulvic acid and humin on the basis of the conventional definitions of these fractions and based on the descriptions of the samples presented in the original publications. The samples were further classified as soil, freshwater, marine, peat and coal

humic materials, depending on their sources. The samples described as marine were obtained almost exclusively from marine sediments; only two fulvic acids and two humic acids in this data set represent dissolved marine materials. Dopplerite, lignite and leonardite humic substances were included with the coal samples. The humic acid set consisted of 410 samples, the fulvic acid set had 214 samples, and there were 26 samples of humin. These samples had been isolated from locations all over the world. Because of the large number of analyses compiled, no single author or publication dominates the contributions to the overall data sets.

The elemental data for each sample consisted of carbon, hydrogen, and oxygen contents in all cases, nitrogen contents for all but two samples, and sulfur contents for approx. 30% of the samples. Only in a few cases had oxygen been determined directly; generally it had been obtained by difference.

Statistical analysis The mean, ($), was computed using the following

equation: tl

~x~

=~=i (1) n

where n is the total number of samples in a set and x~ represents the individual values for a particular parameter. The median (the value about which all other values are equally distributed) and the mode (the most frequently occurring range of values) were also obtained from the data set. The mode was not determined for data sets of <20 samples nor is i t presented for small data sets which exhibited 2 or more intervals of the same frequency. The mean, median and mode are measures of the central ten- dency for that parameter and are the same for normally distributed data.

The range, as presented, gives the low and high values for a parameter in the data set. The standard deviation (s) was calculated using equation (2):

1-- n 2--11/2

s= L. - j . (2)

The range is a measure of the dispersion of the data, and standard deviation is a measure of the dispersion of the data around the mean.

The t-test used here evaluates the probability that the mean value of a particular parameter exhibited by two data sets (.~ and $2) could be observed within the same population. The t-statistic is obtained using equation (3):

t = [ n 2 ( ~ l )J I'`2 (3)

Statistical evaluation of the elemental composition of humic substances 637

where Q is defined by:

r rT Q = n , (x~, - x~,) nl i Ln2J J

[ " l rntll/212 --i~=l(Xl--XEi)L~J .] (4)

and nt and n 2 are the number of measurements in the smaller and larger data sets, and xl, and x2, are the individual values in the smaller and larger data sets, respectively (Crow et aL, 1960). This study employed a one-sided t-test.

Chi-squared (X 2) statistics are used to assess the normality of the distribution of values in the data set for a particular parameter. This information is necessary because the t-test is parametric, i.e. it operates on the assumption that the data exhibit a normal distribution. Consequently, the data in an untransformed state and logm-transformed state were evaluated using the following equation (Crow et al., 1960):

( t / i - ei) 2 X2= (5) / .d i= t ei

where r is the number of categories, r/~ are the observed frequencies and ei are the theoretical frequencies. The form of the data which exhibited, or most-closely approximated, a normal distribution was then used in the t-test. In most instances the data in the untransformed state exhibited, or most-closely approximated, a normal distribution. The use of data transformations in statistical analysis is discussed by Bartlett (1947). Figure 1 gives a series of histograms which illustrate the distribution of the data which was judged to exhibit, or most closely approximate, a normal distribution for three of the data sets used in

this study. A visual assessment of normality can be made for the larger data sets (e.g. all humic acids) but to do so becomes increasingly difficult as the number of samples decreases. Chi-squared statistics are usually a more sensitive indicator of the distribution of the data (Crow et aL, 1960) and for this reason they were used to evaluate the normality of the data when possible. Chi-squared statistics were not calculated for data sets containing < 50 samples. When an assessment of normality was needed for small data sets it was made by inspection of the histograms.

The degrees of freedom (d.f.) were estimated using Satterthwaite's (1946) approximation. This method estimates the degrees of freedom in situations where it is assumed or known that sl :/: s2. The degrees of freedom are obtained using:

r s;T s~+

d.f. = Lnl n2J (6)

nl m n2

Computer programs

The FORTRAN program PHIST (Klusman, 1981) was used to calculate the statistical quantities described in equations (1), (2), and (5). The program orders the values in a data set from lowest to highest which facilitates the determination, by inspection, of the median and range. PHIST also plots the distribution of the data as normal standard deviate histograms from which the mode may be obtained by inspection.

Tests of significance [equation (3)] were performed by TTEST (Klusman et al., 1980), a FORTRAN program for calculating the mean and t-statistic for two sample sets in which it is known, or assumed, t h a t S 1 • S 2 .

C H N

,(::,,,,II,,, ,,,,,dh,,,, ,,,,i,l,,,,, S 0 O/C H/C

.... ,,,.,,,,. . . . . ,,,,,.. . . . . . ,,tl,,,. . . . . . ,,,,It,,... ALL humic acids

( b )

. . . . . . . . , , , , . . . . . . . ,,,,h ,l,, . . . . . . . ,,111,1, .... , ,,,L,, . . . . . ,,llll,,,, .......

Soil humic acids

,,,I II Ih,, ......... ,,,,I I,,, ......

( c )

.... hi,,I . . . . . ,tllh . . . . . I,IIIli . . . . . . . . . , , , . . , . . . . . . till . . . . . . . , , t l . . . . . . h,,lm,.,..,. A

Marine humic acids

Fig. l. Elemental and atomic ratio histograms for (a) unsegregated humic acids, (b) soil humic acids and (c) marine humic acids (based on data in Tables l and 3). All data except the S values are untransformed;

the S data are lOglo-transformed.

638 JAMES A. PaCE and P^TPdCK MACCARTHY

Limitations and assumptions in this study

This study is based on the analysis of experimental data obtained from the literature; as such, the quality of the analytical methodology per se in the individual publications from which the data were obtained cannot be directly evaluated. However, even though the elemental analysis of humic substances is not without its problems (Huffman and Stuber, 1985), carbon and hydrogen determinations can generally be carried out with a high degree of accuracy and precision. It is extremely rare for the results of replicate determinations to be reported in the elemental analysis of humic substances and thus one cannot evaluate the analytical precision pertaining to the individual samples; Huffman and Stuber (1985) in a unique study do, however, report on the precision associated with the elemental analysis of humic substances. For the present study, it must be assumed that the analytical data are accurate and that the dispersion in the data results solely from genuine differences between the samples.

Generally, oxygen content is not determined directly, but is obtained by subtracting the sum of the other elemental contents, plus the ash content, from 100%. As pointed out by Huffman and Stuber (1985) this procedure is subject to two serious limitations: (1) the oxygen content determined in this manner includes the sum of all errors in the other elemental determinations, and (2) the ash may contain elements which have been previously determined, so that subtraction of the ash content, in effect, subtracts those elements a second time. In some studies oxygen has been determined directly (Huffman and Stuber, 1985; Malcolm and MacCarthy, 1986) and generally if the six elements C, H, N, O, S and P are determined, the summation of elemental contents, on an ash-free basis, comes close to 100%, indicating that the organic matter in humic substances is constituted almost entirely from these six elements. However, in the case of some high-ash samples, direct determination of oxygen can result in grossly erroneous values (Malcolm and MacCarthy, 1986; MacCarthy and Malcolm, 1989).

Another problem in the determination of oxygen by difference is that, generally, only C, H and N contents are measured directly, and, consequently, the value calculated for oxygen by subtraction includes other elements such as sulfur and phosphorus. However, as will become evident from the data compiled for this study, sulfur content can represent as much as 3% in some samples of humic substances. In addition, comparatively complete analyses of humic substances (Thurman and Malcolm, 198 I; Huffman and Stuber, 1985) have shown that even phosphorus may consti- tute a measurable fraction of humic substances. For this study the published oxygen contents must be accepted at face value for use in the statistical calcu- lations, but the above limitations relating to the oxygen values should be borne in mind.

The nontrivial problems associated with moisture, and moisture determination, in humic substances are discussed by Huffman and Stuber (1985), who point out that even some commercial laboratories experi- ence difficulty with this problem. The presence of moisture can lead to two types of errors: (1) the hydrogen and oxygen contents of the moisture may be incorrectly attributed to hydrogen and oxygen in the organic matter, and (2) the presence of moisture can lead to an incorrect value for the total organic matter content unless properly corrected for. Again, in this type of study, we cannot assess the extent of these problems, but must assume that the moisture contents were properly determined and corrected for in the source publications, which most probably will not be a correct assumption in some cases. As a result of these considerations, the carbon data may be the least susceptible to error, and conclusions based on the distribution of the element may be the most reliable.

Overall, it is reasonable to assume, based on the results which follow, that the majority of the elemental determinations were carefully conducted and that grossly erroneous values represent only a small fraction of the total sample population and will not seriously influence the statistically-based conclusions of this paper.

RESULTS AND DISCUSSION

Humic substances in general (humic and fulvic acids)

(1) General trends in elemental composition for humic substances. The mean, median, mode, range, standard deviation and X2-statistics for elemental compositions of the humic acids, fulvic acids and humins are given in Table 1. The number of significant figures for each entry in this and subsequent tables is based on the values reported in the source publications. It should be recognized that it is the mean or average values that are being compared in this paper, and that the ranges for elemental contents of various humic fractions, as taken from the literature, actually display considerable overlap (cf. Table 1). Consequently, individual samples may deviate from the general trends that are reported in this paper.

In all data sets the range is defined by the outlier, or extreme, values. Consequently, some of the values specifying the range in Table 1 (and subsequent tables) may appear highly anomalous; e.g. a soil humic acid outlier has a H/C ratio of 0.08, and a soil fulvic acid outlier has a H/C ratio of 2.13 (Table 1). However, these are extreme, and possibly spurious, values and their occurrence is to be expected in a large data set, particularly of the type under discussion here. These extreme values are obviously not representative of the data set as a whole, as evident from comparing them with the corresponding mean values and standard deviations. Table 1 incorporates humic substances from all environments (soil, freshwater,

Tab

le I

. M

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mod

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34

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0.

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(88.

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0.25

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---.

Q

Z2-

stat

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c 11

1.8

(41.

3)

88.8

(41.

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45.3

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22.8

(23.

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97.0

(41.

3)

196.

i (4

1.33

) 49

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1.3)

N

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mpl

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410

410

410

160

410

410

410

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Fu/v

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4.9

2.5*

1.

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45.6

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1.28

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= S

D

5.4

(I 1

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1.0

(20.

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1.6

(64.

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1.2

(100

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0.31

(24

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;t 2-

stat

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1. I

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0.1)

36

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34

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30.1

) 8

No.

of

sam

ples

21

4 21

4 21

4 71

21

4 21

4 21

4 [~

~

t H

um/n

o_

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ean

56.1

5.

5 3.

7 0.

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1.

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5 (6

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7 (4

) 0.

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6 (9

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4.7

(6)

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o

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ge

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7.28

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90-6

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.6)

1.0

(18.

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1.3

(35.

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0.3

(75.

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(9.8

) 0.

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0.5)

E

,~

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atis

tic

..

..

..

..

N

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f sa

mpl

es

26

26

24

16

26

26

26

An

aste

risk

(*)

indi

cate

s th

at t

his

para

met

er i

n th

e hu

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eria

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thes

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640 JAMES A. RICE a n d PATRICK MACCARTHY

marine, peat, coal) and they are not segregated by source. In general, there is close agreement between the mean, median and mode, consistent with the data being normally distributed.

Even though the carbon contents of the humic acids range from 37.18 to 75.76% the standard deviation for those samples is only 5.0% [relative standard deviation (RSD) = 9.1%]. Sixty-eight percent of the humic acids have carbon contents in the interval 50.1-60.1% (+ 1 SD) and 96% of these samples have carbon contents between 45.1 and 65.1% (+ 2 SD). Considering that (1) humic acid is simply an opera- tionally-defined product, (2) the samples involved in this study were obtained by numerous variations of the traditional extractive technique, (3) that these ht/mic acids were extracted by many different exper- imentalists, and (4) that these humic acids represent a broad diversity of source environments (soil, freshwater, marine, peat and coal), geographical locations and botanic precursors, the absolute standard deviation of only 5.0% (9.1% RSD) is noteworthy. In fact, it was precisely this recognition of suprisingly small standard deviations, here and elsewhere in this study, that prompted us to embark on a more rigorous statistical evaluation of the elemental compositions of humic substances. This observation may suggest that an optimum composition exists for humic acids in nature. This, however, does not imply the existence of molecular structural uniformity among humic substances.

The range and relative standard deviation for the carbon contents offulvic acids are somewhat larger than those for humic acids, and the corresponding values for humin are considerably smaller (Table 1). However, one cannot speak as reliably of the humin data because only 26 such samples were involved in the study, and more than half of these were from peat; these limited data may not be representative of humin in general.

The mean values for the H/C and O/C atomic ratios based on these large data sets are similar to those previously reported for humic acid and fulvic acid (Visser, 1983; Steelink, 1985) and for humin (Stuermer et aL, 1978) based on much smaller data sets. The H/C ratio (Table 1) indicates that fulvic acids are, in general, more aliphatic than humic acids.

It is seen from Table 1 that the ranges in reported carbon contents for humic acids and fulvic acids are similar. However, a single outlying datum can have an inordinate influence on the range, but the mean carbon contents for humic and fulvic acids are considerably different, displaying the net differences between these materials.

The ranges for other elemental contents are also rather large (Table 1). The relative standard deviations for hydrogen are moderately high, for nitrogen are quite large and for sulfur are very large (88.8% for humic acid and 83.3% for fulvic acid). The large standard deviations for nitrogen and sulfur may simply be a result of inherent variability of these

Table 2. t-Statistics, and their significance, used to compare unseg- regnted humic acids (HA), fulvic acids (FA) and humins (HMN)

from Table I

t-Statistic

t c C H N O O/C H/C

H A - F A 3.29 19.66" 2.08 6.47* 21.41" 20.03* 7.67* H A - H M N 3.58 1.64 2. I 1 1.48 I. 17 2.51 1.43 F A - H M N 3.48 17.68" 2.80 2.76 12.54" 8.75* 1.51

t c Represents the critical t-value at P = 0.0005 for each comparison and an asterisk (*) indicates a t-value significant at P < 0.0005.

elements in humic substances, and may also be due to analytical errors in the determination of these lesser constituents. On the basis of these data, and for the reasons stated in the previous section, conclusions based on observed trends in the means may be most reliable in the case of carbon.

(2) Statistically significant differences in humic substances. Numerous trends can be discerned in the data of Table 1. The importance of such relatively large data sets is that they allow the statistically significant trends to be established by means of the t-test. Table 2 presents the t-statistics, calculated using equation (3), which compare humic acid with fulvic acid, humic acid with humin, and fulvic acid with humin for each of the elemental contents and atomic ratios of the unsegregated samples described in Table 1. Due to the comparatively small number of sulfur analyses, the statistical significance of trends observed in the sulfur contents are not evaluated. The probability, P, at which to reject the null hypothesis equating the means of a parameter exhibited by two data sets has been set at P < 0.0005. This means that if the t-statistic for a particular parameter exceeds the critical value (to) there is less than one chance in 2000 that the means exhibited by those two data sets could be observed in the same population; the values that fall into this category are labeled with an asterisk in Table 2. Many of the t-values in Table 2 are very much larger than the critical values, to; in these cases the null hypothesis is rejected at P-values that are considerably less than 0.0005.

Fulvic acid is statistically distinct from humic acid on the basis of its C (lower), N (lower) and O (higher) contents and its O/C ratio (higher). Fulvic acid also exhibits a larger and statistically different H/C ratio relative to humic acid. Because a larger H/C ratio is indicative of a more aliphatic character, this is consistent with fulvic acid being, in general, more aliphatic than humic acid. The only parameter for which fulvic acid is not significantly different from humic acid in Table 2 is hydrogen content.

Fulvic acid differs statistically from humin with respect to its C (lower) and O (higher) contents and its higher O/C ratio.

The statistical comparison of humic acid and humin does not show these two materials to be significantly different at P < 0.0005, and cannot dis- count the possibility that the parameter means exhibited by the respective data sets in this study might be observed in the same population at the chosen

Tab

le 3

. M

ean,

med

ian,

mod

e, r

ange

, st

anda

rd d

evia

tion

(SD

) an

d X

2-st

atis

tics

for

elem

enta

l com

posi

tion

s of

hum

ic a

cids

fro

m d

iffe

rent

sou

rces

exp

ress

ed a

s w

eigh

t pe

rcen

t;

O/C

and

H/C

rat

ios

are

atom

ic p

erce

nt.

All

val

ues

are

on a

n as

h-fr

ee b

asis

C

H

N

S 0

O/C

H

/C

Soil

hum

ic a

cids

M

ean

55.4

4.

8 3.

6 0.

8*

36.0

0.

50*

1.04

M

edia

n 56

.0

4.9

3.7

0.6

35.4

0.

47

1.06

M

ode

56.3

-57.

3 (3

8)

5.0°

5.3

(32)

3.

9-4.

2 (3

0)

0.5-

0.6

(14)

35

.0-3

6.0

(34)

0.

47-0

.49

(44)

1.

04-1

,10

(38)

R

ange

37

.18-

64.1

1.

64-8

.0

0.50

07.0

0 0.

1-4.

88

27.1

-51.

98

0.33

-0.9

7 0.

08-1

.77

SD

3.

8 (6

.9)

1.0

(20.

8)

1.3(

36. l

) 0.

6 (7

.5)

3.7

(10.

3)

0,09

(18.

0)

0.25

(24.

0)

X 2-

stat

isti

c 72

.1 (

30.1

) 51

.6 (3

0.1)

39

.7 (3

0.1)

9.

2 (9

.5)

55.7

(30.

1)

71,7

(30

.1)

41.2

(30.

1)

No.

of

sam

ples

21

5 21

5 21

5 67

21

5 21

5 21

5

Fre

shw

ater

hum

ic a

cids

M

ean

51.2

4.

7 2.

6*

1.9*

40

.4

0.60

1.

12

Med

ian

5 I.

7 4.

7 2.

1 1.

1 40

.8

0.60

1.

12

Mod

e 51

.2-5

1.9(

7)

4.7-

-4.9

(8)

1.9-

2.2(

12)

0.6-

2.0(

8)

40.4

-41.

4(12

) 0.

56-0

.58(

10)

1.12

-1.1

6(9)

R

ange

43

.75-

56,0

0 3.

5-6.

54

0.63

-7.9

7 0.

35-4

.31

30.8

6-48

.2

0.42

-0.8

0 0,

79-1

,69

SD

3.

0 (5

.9)

0.6

(12.

8)

1.6

(61.

5)

1.4

(73.

7)

3.8

(9.4

) 0.

08 (1

3.3)

0.

17 (1

5.2)

X

2-st

atis

tic

1.3

(7.8

) 2.

0 (7

.8)

5.6

(7.8

) 5.

6 (7

.8)

3.5

(7.8

) 3.

5 (7

.8)

No.

of

sam

ples

56

56

56

13

56

56

56

Mar

ine

hum

ic a

cids

M

ean

56.3

* 5.

8*

3.8

3.1"

31

,7

0.45

1.

23

Med

ian

56.4

5.

5 3.

7 2.

6 32

.1

0.43

1.

20

Mod

e 55

.9-5

7.6

(21)

5.

1-5.

4 (1

4)

3.0-

3.8

(26)

2.

2-2.

5 (9

) 31

.8-3

5.6

(34)

0.

40-0

.45

(23)

1.

05-1

.11

(12)

R

ange

37

.52-

75.7

6 3.

76-1

1.68

0.

9%10

.54

1.2-

8.3

7.93

-56.

6 0.

08-1

.20

0.6%

1.85

S

D

6.6

(11.

7)

1.4

(24.

1)

1.5

(39.

5)

1.4

(45.

2)

7.8

(24.

6)

0.18

(40.

0)

0.23

(18.

7)

X2-

stat

isti

c 14

.0 (1

4.1)

14

.4 (1

4.1)

6.

5 (1

4.1)

3.

3 (9

.5)

23.7

(14.

1)

33.9

(14.

1)

7.8

(14.

1)

No.

of

sam

ples

95

95

95

66

95

95

95

Pea

t hu

mic

aci

ds

Mea

n 57

,1

5.0

2.8

0.4*

35

.2

0.47

1.

04

Med

ian

57.4

5.

0 2.

9 0.

3 34

.5

0.46

1.

06

Mod

e 57

.7-5

8.4

(5)

5.0-

5.2

(4)

3.3-

3.5

(5)

1.9-

2.1

(5)

35.8

-36.

5 (5

) 0.

47-0

.48

(7)

1.04

-1.1

3 (6

) R

ange

50

.53-

62.7

5 3.

6-6.

57

0.60

-3.9

0,

1-0.

7 30

.68-

43.2

0 0.

37-0

.64

0.73

-1.3

5 S

D

2.5

(4.4

) 0.

8 (1

6.0)

1.

0 (3

5.7)

0.

2 (5

0.0)

2.

7 (7

.7)

0.06

(12.

8)

0.17

(16.

3)

X 2-

stat

isti

c .

..

..

..

N

o. o

f sa

mpl

es

23

23

21

12

23

23

23

e~

I~'.

o E

8 o

An

aste

risk

(*)

indi

cate

s th

at t

his

para

met

er i

n th

is h

umic

aci

d ex

hibi

ted

or m

ost-

clos

ely

appr

oxim

ated

a l

og-n

orm

al d

istr

ibut

ion;

all

oth

er v

alue

s ex

hibi

ted

a no

rmal

dis

trib

utio

n.

Val

ues

in p

aren

thes

es f

ollo

win

g th

e m

ode

are

the

num

ber

of s

ampl

es i

n th

at i

nter

val,

tho

se f

ollo

win

g th

e st

anda

rd d

evia

tion

are

rel

ativ

e st

anda

rd d

evia

tion

s (R

SD).

x2

-Sta

tist

ics

are

prov

ided

for

the

mos

t no

rmal

for

m o

f th

e da

ta a

s in

dica

ted

abov

e an

d X

2 fo

r th

at s

ampl

e si

ze f

ollo

ws

in p

aren

thes

es.

m

642 JAbl~ A. RICE and PATRICK MACCARTHY

probability level. This could be the result of real similarities in the average elemental compositions of humic acid and humin (on an ash-free basis) or it could be due to the small number of analyses from which the humin data set was constructed. A small data set tends to diminish the ability of statistical devices, such as the t-test, to discern subtle differences. Of course, humic acid and humin differ considerably in other characteristics such as ash content, solubility, and so on.

Humic acids segregated by source

(I) General trends in elemental composition for humic acids. Table 3 contains the statistical data for humic acids where the samples are segregated according to source: soil, freshwater, marine, and peat. Coal-derived humic acids, which were included in Table 1, are not included in this classification as there were too few data to make the presentation of average elemental compositions meaningful for this category or to perform tests of statistical significance. The distribution of the data, on the basis of X2-stat - istics, more closely approximates normality when the humic acids are segregated by source (Table 3) than when they are considered in toto (Table 1).

The most striking observation on comparing the data in Table 3 with those in Table 1 is that when the humic acids are segregated by source, the range of carbon values narrows dramatically and the standard deviations for carbon decrease remarkably except for the marine humic acid, where only a small decrease in the range occurs and an increase in the relative standard deviation is observed. For example, the standard deviations for carbon contents decrease by about 25, 40 and 50% for soil, freshwater and peat humic acids, respectively, compared to the standard deviation of humic acids considered in toto. This observation, in conjunction with the noticeably different averages for humic acids from different sources (Table 3) suggests a "preferred" composition for the humic acids from each environment.

The fact that the standard deviations do not con- tract in the case of the marine humic acids may result from the variety of environments (near-shore, deep- sea, etc.) in this study which were classified by the original authors as being marine, some of which have the likelihood of several sources of organic matter input. For example, a near-shore marine environment may receive organic matter from terrestrial as well as algal sources due to its proximity to land masses.

Table 4. t-Statistics, and their significance, used to compare humic acids from soil, freshwatvr and marine sources from Table 2

t-Statistic

t~ C H N O O/C H/C

Soil-freshwater 3.39 8.98* 0.19 6.07* 8.43* 8.38* 2.68 Soil-marine 3.40 0.79 7.37* 0.71 4.83* 3.70* 6.01" Freshwater-

marine 3.29 6.56* 8.80* 5.01" 10.82" 10.15" 4.14"

tc Represents the critical t-value at P = 0.0005 for each comparison and an asterisk (*) indicates a t-value significant at P < 0.0005.

There is a similar pronounced contraction (except for the marine samples) of the standard deviation for oxygen contents when the humic acids are separated by source. This is also generally true for the standard deviations of S, N and H (with some exceptions) though the effect is not so pronounced.

The larger H/C ratio (Table 3), consistent with a more aliphatic nature, for marine humic acid relative to soil humic acid is in agreement with previous reports based on considerably smaller data sets (Ishi- watari, 1969; 1970; Rashid and King, 1970; Stuermer and Payne, 1976; Steelink, 1985). The average elemental composition of soil humic acid in Table 3 falls within the range of values reported for an "average" soil humic acid by Schnitzer (1977) based on a substantially smaller data set.

(2) Statistically significant differences in humic acids. Table 4 presents the statistical significance of differences in the means of the elemental components of humic acid isolated from different sources. Peat humic acid was not involved in this comparison because of the comparatively small number of data in this set. Table 4 shows that humic acid obtained from freshwater sources possesses an elemental composition distinct from soil and marine humic acids. Marine humic acid is statistically different from soil humic acid with respect to hydrogen and oxygen contents and the H/C ratio. The O/C ratio suggests that real differences exist between freshwater, soil and marine humic acids.

A statistical distinction may be made between marine humic acid and freshwater and soil humic acids on the basis of the H/C ratio. A similar distinction cannot be made between soil and freshwater humic acids at the chosen probability level, although they are statistically different at a lower level, P < 0.005. This lower probability level still allows only one chance in 200 that the means could be observed in the same population, and suggests that the lack of statistical distinction between the freshwater and soil humic acids at P < 0.0005 may be due to the small number of analyses of the freshwater humic acid relative to the soil humic acid. Thus, it is probably reasonable to infer that the aliphaticity of humic acids from different sources decreases in the order marine > freshwater > soil. This would be consistent with the nature of the organic input into soil and marine environments. Marine organic detritus is believed to be predominantly algal in origin and of a highly aliphatic nature (Nissenbaum and Kaplan, 1972) while soil organic input is believed to be more aromatic in nature due to the ubiquity of lignin in terrestrial plants (Flaig, 1972). A freshwater sample could have major inputs of either algal or soil organic materials. In summary, the data in Table 4 indicate that there are statistically significant differences between the elemental contents of humic acids from soil, freshwater and marine sources.

Tab

le 5

. M

ean,

med

ian,

mod

e, r

ange

, st

anda

rd d

evia

tion

(SD

) an

d X

2-st

atis

tics

for

ele

men

tal

com

posi

tion

s of

ful

vic

acid

s fr

om d

iffe

rent

sou

rces

exp

ress

ed a

s w

eigh

t pe

rcen

t,

O/C

and

H/C

rat

ios

are

atom

ic p

erce

nt.

All

val

ues

are

on a

n as

h-fr

ee b

asis

C

H

N

S O

O

/C

H/C

Soil

fuM

c ac

ids

Mea

n 45

.3

5.0

2.6

1.3

46.2

0.

78

1.35

M

edia

n 44

.2

5.0

2.8

0.9

47.1

0.

79

1.33

M

ode

39.9

-41.

3(17

) 4.

8-5.

0(18

) 0.

7-1.

0(18

) 0.

2~).

4 (1

3)

43.6

-44.

9(19

) 0.

824)

.86(

17)

1.00

-1.0

9(16

) R

ange

35

.1-7

5.4

3.2-

7.00

0.

45-5

.87

0.1-

3.6

16.9

-55.

88

0.17

-1.1

9 0.

77-2

.13

SD

5.

4 (1

1.9)

1.

0 (2

0.0)

1.

3 (5

0.0)

1.

1 (8

4.6)

5.

2 (1

1.3)

0.

16 (

20.5

) 0.

34 (2

5.2)

Z

2-st

atis

tic

36.7

(18.

3)

13.3

(I 8

.3)

33.5

(18

.3)

--

20.5

(18.

3)

18.8

(18.

3)

4.8

(18.

3)

No.

of

sam

ples

12

7 12

7 12

7 45

12

7 12

7 12

7

Fre

shw

ater

fulv

ic a

cids

M

ean

46.7

4.

2 2.

3 *

1.2

45.9

* 0.

75

I. 1

0*

Med

ian

46.2

4.

3 1.

8 1.

0 46

.8

0.76

1.

10

Mod

e 44

.5-4

5.6(

11)

4.2

~.4

(13

) 1.

8-2.

3(16

) --

48

.3--

49.7

(10)

0.

78-0

.81

(8)

1.09

-1.1

2(11

) R

ange

39

.2-5

6.33

0.

43-5

.9

0.47

-8.1

6 0.

1 ~

3.0

5

34.6

6-55

.8

0.49

-1.0

7 0.

81-1

.53

SD

4.

3 (9

.2)

0.7

(16.

7)

2.1

(91.

3)

0.9

(75.

0)

5.1

(11.

1)

0.14

(18.

7)

0.13

(11

.8)

12-s

tati

stic

8.

9 (9

.5)

12.9

(9.5

) 20

.5 (9

.5)

--

8.9

(9.5

) 4.

3 (9

.5)

3.2

(9.5

) N

o. o

f sa

mpl

es

63

63

63

14

63

63

63

Mar

ine

fulv

ic a

cids

M

ean

45.0

5.

9 4.

1 2.

1 45

.1

0.77

1.

56

Med

ian

45.4

6.

l 4.

5 --

44

.5

0.73

1.

62

Mod

e .

..

..

..

R

ange

38

.4-5

0.00

4.

34i.

80

1.0~

.83

--

36.9

-54.

5 0.

55-1

.07

1.31

-1.7

3 S

D

4.0

(8.9

) 0.

9 (1

5.3)

2.

3 (5

6.1)

--

6.

0 (1

3.3)

0.

17 (

22.1

) 0.

13 (8

.3)

X2.

staf

isti

c .

..

..

..

N

o. o

f sa

mpl

es

12

12

12

1 12

12

12

Pea

t ful

vic

acid

s M

ean

54.2

5.

3 2.

0 0.

8 37

.8

0.53

1.

20

Med

ian

54.1

4.

9 2.

2 0.

6 38

.8

0.54

1.

04

Mod

e .

..

..

..

R

ange

46

.9-6

0.8

4.2-

7.2

1.2-

2.6

0.2-

1.9

31.1

-44.

3 0.

38-0

.71

0.85

-1.8

4 S

D

4.3

(7.9

) 1.

1 (2

0.8)

0.

5 (2

5.0)

0.

6 (7

5.0)

3.

7 (9

.8)

0.09

4 (1

7.0)

0.

33 (

27.5

) Z

2 st

atis

tic

..

..

..

.

No.

of

sam

ples

12

12

12

11

12

12

12

go

o 8 ~°

o o ~r

B"

ga

m

An

aste

risk

(*)

ind

icat

es t

hat

this

par

amet

er in

thi

s fu

lvic

aci

d ex

hibi

ted

or m

ost-

clos

ely

appr

oxim

ated

a l

og-n

orm

al d

istr

ibut

ion;

all

oth

er v

alue

s ex

hibi

ted

a no

rmal

dis

trib

utio

n.

Val

ues

in p

aren

thes

es f

ollo

win

g th

e m

ode

are

the

num

ber

of s

ampl

es i

n th

at i

nter

val,

tho

se f

ollo

win

g th

e st

anda

rd d

evia

tion

are

rel

ativ

e st

anda

rd d

evia

tion

s (R

SD

).

x2-S

tati

stie

s ar

e pr

ovid

ed f

or t

he m

ost

norm

al f

orm

of

the

data

as

indi

cate

d ab

ove

and

X 2

for

that

sam

ple

size

fol

low

s in

par

enth

eses

.

644 J ~ s A. RIcE and PATmCK MACCAItTm¢

Fulvic acids segregated by source

(1) General trends in elemental composition for fulvic acids. Table 5 contains the statistical data for fulvic acids segregated by source. Grouping fulvic acids by source causes a significant narrowing of the range and standard deviation of carbon values, except in the case of the soil samples where the range is unaltered and the relative standard deviation is virtually un- changed. The ranges and standard deviations for most other parameters also decrease when the fulvic acids are segregated by source although there are exceptions to this generalization.

Segregation of the fulvic acid samples by source (Table 5) results in the data more closely approximat- ing a normal distribution than when those data are evaluated in toto (Table 1). The most normal form of the data in Table 5, in contrast to that in Table 1, exhibits Z2-statistics very near or below the critical values, confirming that these data exhibit or very closely approximate a normal distribution.

Compared to soil, freshwater and marine fulvic acids, all of which exhibit similar carbon and oxygen contents, peat fulvic acid displays markedly different values for C (higher), O (lower) and consequently, a lower O/C ratio. Hydrogen and H/C values for peat fulvic acid fall within the range of values exhibited by other samples. However, caution must be exercised in attempting to draw conclusions from these observations in that only twelve samples of peat fulvic acid are involved in this study and eleven of these were collected by a single group from different areas of one locale (Zelazny and Carlisle, 1974). This may introduce some bias into the estimate of this sample population mean, and consequently peat fulvic acid will not be discussed any further in this paper.

Comparing the soil, freshwater and marine "average" fulvic acids in Table 5 reveals that the C and O contents and the O/C ratios are quite similar. Though not as pronounced as when humic acids are segregated by source, the H, N and S contents of the fulvic acids do show different averages, when grouped by source, compared to the average of all fulvic acids combined. On the basis of the H/C ratio the "average" soil fulvic acid would appear to be more aliphatic than freshwater fulvic acid.

(2) Statistically significant differences in fulvic acids. Table 6 reveals that the only statistically significant difference between fulvic acids from soil and freshwater environments is in the hydrogen content and in the H/C ratio. Because of the small number of marine and peat fulvic acids for which data were available, they were excluded from this type of comparison. By

Table 6. t-Statistics, and their significance, used to compare fulvic acids from soil and freshwater sources from Table 3

t -Statistic t c c H N O O/C H/C

Soil-freshwater 3.29 2.56 6.88* 2.23 0.30 2.04 7.18" t, Represents the critical t-value at P = 0.0005 and an asterisk (*)

indicates a t-value significant at P < 0.0005.

Table 7. t-Statistics, and their significance, used to compare humic acids (HA) and fulvic acids (FA) from similar (soil and freshwater)

sources

t -Statistic

t¢ C H N O O/C H/C Soil HA-FA 3.29 16.75" 2.19 8.24* 17.85" 17.41' 8.36* Freshwater

HA-FA 3.38 6.35* 4.35* 2.47 6.04* 6.42" 0.72 t c Represents the critical t-value at P = 0.0005 and an asterisk (*)

indicates a t-value significant at P < 0.0005.

inference the~, soil fulvic acid is more aliphatic than freshwater fulvic acid due to its larger H/C ratio. This result is interesting because it has often been stated that soil humic materials are generally more aromatic than humic substances from other environments. For example, Visser (1983) observed that humic materials from freshwater sources generally exhibited H/C ratios greater than that of soil humic substances. This apparent difference is usually attributed to the ubiquity of lignin, which some believe to be an important humic precursor, in terrestrial plants. The H/C ratio for freshwater humic acid is greater than that for soil humic acid (Table 1) but the difference is not statistically significant at P < 0.0005 (Table 5).

Intercomparison of humic and fulvic acids from similar environmen ts

(1) General trends in elemental composition. When segregated by source, fulvic acids (Table 5) consist- ently exhibit a lower C content, higher O content and a higher O/C ratio than do humic acids (Table 3) from the same type of environment. No clear distinctions between humic acid and fulvic acid from similar environments are evident when trends in the H, N

2.5

2.0

o

1.5

"1-

i ~.o

0..5 HA ".%',%

%, fYiTI FA HMN

o I 0.5 1.0 1.5

A t o m i c O/C r o t i o

Fig. 2, van Krevelen diagram based on humic acid (410 samples), fulvic acid (214 samples) and humin (26 samples).


and S contents are examined. The fulvic acids, with one exception, exhibit a greater H/C ratio than the respective humic acids.

(2) Statistically significant differences between humic and fulvic acids from similar environments. Visser (1983) concluded that the majority of fulvic and humic acid samples from freshwater environments exhibit similar H/C ratios. The results of the t-test in Table 7 support this observation by indicating that the differences in the means of the H/C ratios in freshwater fulvic acid and humic acid are not significant. How- ever, freshwater humic acid and fulvic acid differ significantly from each other with respect to the other parameters in Table 7, except for the nitrogen content.

Table 7 also corroborates statements in the literature (Kononova, 1966; Schnitzer and Skinner, 1974) which point out that soil fulvic acid (Table 5) is more aliphatic than soil humic acid (Table 3). Preston and Ripmecster (1982) report a "predominantly aliphatic" nature for soil fulvic acid based on ~3C-NMR spectra. Soil fulvic acid also exhibits a statistically different weight-percent, with respect to soil humic acid, for every element except hydrogen (Table 7) emphasizing that these two soil humic substances differ in more ways than just their degree of aliphaticity.

Because of the small total number of humin samples in the data set (Table 1), humin was not segregated by source.

van Krevelen diagrams

Figure 2 shows a van Krevelen diagram based on the 650 samples in Table 1, and illustrates the compo-

sitional fields occupied by humic acid, fulvic acid and humin. In this paper van Krevelen diagrams are used, as elsewhere (Kuwatsuka et al., 1978; Visser, 1983), only to summarize compositional differences between humic materials. Humic and fulvic acids occupy broad compositional fields with a considerable region of overlap. The humin samples occupy a compositional field that lies almost completely within that of humic acid.

The "thumb" extending from the humic acid field into the upper left-hand portion of Fig. 2 represents 11 marine humic acids extracted from Eocene chalks and cherts in the investigation of Bein and Sandier (1983). These samples exhibit H/C ratios which are not markedly different from others in the humic acid data set; the O/C ratios of these samples, however, represent the lower extreme in the range of O/C ratios listed in Table 1 showing values of about 0.20 or less. Even though these 11 humic acids were extracted by a rather conventional sodium hydroxide-sodium pyrophosphate procedure (Bein and Sandier, 1983), and consequently conform to the operational defi- nition of humic acid, some obvious differences exist between them and the bulk of the humic acids represented in this diagram.

The van Krevelen diagram presented in Fig. 2 differs from that of Kuwatsuka et al. (1978) in three ways: (1) this diagram includes data for humin samples, (2) approximately ten times as many humic acid samples and thirty times as many fulvic acid samples are utilized in the present diagram and (3) Kuwatsuka et al. (1978) described the distribution of

2.5 2.5

2.0

o . . . . . .

• 222 "~,~:~i :*: ~ : • • .... ~s*; t.s*. :'o

._o : . ~ ~ . . . 1.0 , ...,.~: . . . . . ..:::!:;::::.

o.5 ~ ii~ E~ M

~ s r r rnp

I I 0.5 1.0 1.5

A t o m i c O / C r o t i o

Fig. 3. van Krevelen diagram based on soil humic acids (215 samples), freshwater humic acids (56 samples), marine sediment humic acids (95 samples) and peat humic acids (23

samples).

2.0

o

E 1.5

-r

E 0 1.0

0 . 5 ffT~lM

~s FTmp

I I 0.5 1.0 Atomic O/C rotio

1.5

Fig. 4. van Krevelen diagram based on soil fulvic acids (127 samples), freshwater fulvic acids (63 samples), marine sediment fulvic acids (12 samples) and peat fulvic acids

(12 samples).

646 JAMES A. RICE and PATRICK MACCARTHY

the humic acids on their van Krevelen diagram as being '"J"-shaped', but such a description is not applicable to the larger data set employed in the construction of Fig. 2 even though the data of Kuwatsuka et al. are included in this figure. In general, however, the regions occupied by humic and fulvic acids are similar to those of Kuwatsuka et al. (1978). The considerably larger data base used in the present study enhances the reliability of conclusions based on these diagrams.

Tissot and WeRe (1978) have described humic substances as being similar to Type III kerogens. In terms of Fig. 2 and the data in Table 1 (moderate H/C ratios, high O/C ratios), this appears reasonable; the average humic material exhibits atomic ratios typical of Type III kerogens.

Figures 3 and 4, which are van Krevden diagrams for humic acids and fulvic acids, respectively, segregated by source, emphasize the difference between humic materials from different environments that were described in earlier sections. Figure 3 shows that freshwater, peat and marine humic acids exhibit considerable overlap in their compositions. The soil humic acid compositional field overlaps that of the other three. The compositional fields occupied by freshwater, marine and peat fulvic acids (Fig. 4) appear to be mutually more distinct than their humic acid counterparts (Fig. 3). Such apparent distinctions may not be evident if the marine and peat fulvic acid data sets were larger. As with humic acids segregated by source (Fig. 3), soil fulvic ac;d extensively overlaps the compositional fields of the other three fulvic acids in Fig. 4.

Average elemental compositions o f artificial and sewage sludge "humic " materials

Finally, in addition to the natural humic substances already discussed in this paper, the elemental data for a very limited number of artificial and sewage-sludge derived "humic materials" were also briefly examined for comparative purposes. The mean values for the dcmental contents and atomic ratios of 17 microbially-produced "humic acids", 3 chemically synthesized "humic acids", and 3 sewage- sludge "fulvic acids" are given in Table 8. The microbially-produced "humic acids" have mean values similar to those for unsegregated humic acids (Table 1). For the chemically synthesized "humic acids" the O/C ratio is similar to, and the H/C ratio is less than that of unsegregated humic acids. Only one of these three "humic acids" contained nitrogen (at a very high level). Chemically synthesized humic

Table 8. Mean elemental contents (weight pcrcant) and atomic ratios for i 7 microbially produced "humic acids", 3 chemically synthesized "humic acids" and 3 sewage sludge "fulvic acids". All values an: on

an ash-free basis

Sample C H N S O O/C H/C

Microbial 52.6 5.1 4.1 - - 38.1 0.54 1.16 Chemical 58.1 3.2 3.5 -- 37.4 0.48 0.66 Sewage-sludge 40.82 6.57 2.83 8.15 42.27 0.78 1.93

acids do not contain nitrogen unless it is present in one of the reagents. The O/C ratio for the sewage sludge "fulvic acid" is similar to that of the unsegregated fulvic acids (Table 1) while the H/C value is considerably greater. The sewage sludge "fulvic acids" have a very high sulfur content possibly due to the presence of sulfonated detergents; all three of these "fulvic acids" come from the same study (Sposito et al., 1976). Differences between commercial humic acids and soil and water humic substances have also been reported (Malcolm and MacCarthy, 1986; MacCarthy and Malcolm, 1989).

SUMMARY

The statistical analysis presented in this paper quantitatively identifies similarities and differences between various humic substances, and in addition to establishing new trends in the properties of these materials, it provides a statistical basis for some previously-reported trends.

The results of this study are summarized as follows:

(1) Humic acid, fulvic acid and humin exhibit surprisingly small standard deviations in their elemental contents, particularly for carbon, even for unsegregated samples. This may suggest that some preferred composition, or a relatively narrow range of compositions, exists for humic substances in nature.

(2) The t-test indicates that the C, N, and O contents and the O/C and H/C ratios exhibited by fulvic acid are significantly different from those of humic acid at P < 0.0005. The carbon and oxygen contents and the O/C ratio of fulvic acid are statistically different from those of humin. No statistically significant differences in the elemental composition exist, on the basis of these data sets, between humin and humic acid.

(3) When segregated by source, humic acids from different environments exhibit different means in the elemental parameters; considerable decreases in standard deviations compared to humic acids in toto are evident.

(4) Freshwater humic acid exhibits statistically significant differences in its C, N and O contents and O/C ratio compared to marine and soil humic acids. Marine humic acids display statistically significant differences in their H and O contents and in the O/C and H/C ratios compared to soil humic acids. In addition, two of the three humic acid source inter- comparisons (marine-soil and marine-freshwater) exhibit statistically different H/C ratios. By inference, the degree of aliphaticity of humic acids decreases in the order marine > freshwater > soil.

(5) Variations in means of the fulvic acids, when segregated by source, are not as pronounced as for humic acids, but in most cases there is still a considerable decrease in the standard deviations compared to unsegregated fulvic acids.


The dimunition of the standard deviation which occurs when the samples are segregated by source, coupled with the statistically significant differences in the mean indicates, perhaps, that there is an optimum compositional range to be expected for each humic material in a particular environment.

(6) Statistically distinct H contents and H/C ratios exist in fulvic acids from soil compared to freshwater source environments. Because the H/C ratio exhibited by the soil fulvic acids is larger, it is assumed to be more aliphatic than freshwater fulvic acid.

(7) Soil fulvic acid exhibits statistically significant differences in the C, N and O contents and O/C and H/C ratios when compared to those of soil humic acid.

(8) A van Krevelen diagram based on 650 samples of humic acids, fulvic acids and humins shows that humic and fulvic acids occupy distinct regions on the diagram with an area of overlap. The humin compositional field exists almost completely within the humic acid field.

(9) A van Krevelen diagram based on humic acids segregated by source shows considerable overlap between the compositional fields of soil, freshwater, marine and peat humic acids. A van Krevelen diagram based on fu lv ic acids segregated by source shows that soil fulvic acid overlaps the compositional fields of freshwater, marine and peat fulvic acids. Freshwater, marine and peat fulvic acids appear to exhibit distinct compositional fields on the diagram.

(10) While chemically synthesized "humic acids" and sewage sludge "fulvic acids" conform to the operational definitions of these materials their minor elemental components indicate differences between them and naturally occurring humic substances. Sewage sludge "fulvic acids" and chemically synthesized "humic acids" exhibit some pronounced differences in their S and N contents, respectively, compared to "natural" humic materials. Microbially produced humic acids are generally similar to the humic acid data set in toto.

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statistical evaluation of elemental composition of humic substances

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